a. Field of the Invention
The present invention relates generally to automatic tank gauging (ATG) systems that use an acoustic or ultrasonic sensor system to measure level and temperature, as well as changes in level and temperature, in liquids stored in underground and aboveground tanks; it applies particularly to novel reference subsystems and methods for monitoring (1) the level of that liquid, (2) small changes in the level and temperature of that liquid, (3) the level of a second liquid that is immiscible with the first liquid and that is usually located near the bottom of the tank, and (4) leakage from the tank.
b. Brief Discussion of the Prior Art
Automatic systems for making level and volumetric measurements in storage tanks are well known in the petroleum and chemical industries and are generally included, under the category of automatic tank gauging systems, in the United States Environmental Protection Agency (EPA) regulation for underground storage tanks containing petroleum liquids and other chemical liquids considered hazardous to the environment. Petroleum and chemical liquids are referred to as "product" in order to differentiate them from water, another liquid that may be present in the tank at the same time (water, which accumulates at the bottom of the tank, is an undesired entity).
The EPA presently gives three options for tank testing. The first option is an ATG, which must (1) do inventory control (i.e., make water-level and product-level measurements accurate to 1/8 in.) and (2) perform a leak detection test (i.e., detect leaks as small as 0.2 gal/h with a probability of detection (P.sub.D) of 95% and a probability of false alarm (P.sub.FA) of 5%). The ATG reconciles product inventory on a monthly basis, using the product-level measurements combined with dispensing and delivery data obtained by other means. In addition, the ATG must be used monthly to perform a leak detection test. The second option is a volumetric or "tank tightness" test, which, like an ATG, must make inventory control measurements of water level and product level accurate to 1/8 in. and must perform a leak detection test. It must, however, be able to detect leaks twice as small as those detectable by an ATG, specifically, 0.1 gal/h with a P.sub.D of 95% and a P.sub.FA of 5%. If one chooses this more stringent option, the minimum time interval between tests required by the regulation is once a year. The EPA specifies a third option under Other Methods in the regulatory document; to satisfy this option one must test the tank monthly with any method that can detect a release of 0.2 gal/h using a P.sub.D of 95% and a P.sub.FA of 5%.
An ATG can be used to satisfy any of these three options as long as it meets the criteria defined for that option by the EPA. Moreover, the present invention, when used for leak detection, may be used either as an ATG or a tank tightness test.
A wide variety of ATGs have been developed and are currently on the market. A few of these use acoustic systems to measure product level and water level, both for inventory control and for conducting a leak detection test. The acoustic frequency selected for the system is such that an acoustic pulse will propagate in a liquid but not in a gas and will be reflected from any strong density discontinuity, such as the interface between a liquid and a gas, the interface between two immiscible liquids (e.g., gasoline and water), or a solid object (e.g., brass, steel, or nylon). When the interface is between a gas and a liquid, almost all of the acoustic energy is reflected; with a liquid-liquid interface, or a reflecting target (known as a "fiducial") that is narrower than the acoustic beam, some of the acoustic energy is reflected and some continues to propagate toward the liquid surface (in most systems, the acoustic transducer is placed on or near the bottom of the tank and pointed upward). If the average speed of sound over the propagation path is known, the height of the liquid above the transducer can be estimated from the time it takes for an acoustic pulse to travel back and forth once between the transducer and the product surface. The speed of an acoustic pulse through liquids such as those found in storage tanks depends on the temperature and chemical composition of the liquid. For a given liquid, the speed is directly proportional to the temperature over the range of ambient product temperatures, which is similar to the range of ground and air temperatures. Thus, the round-trip travel time between the transducer and the surface will depend on the vertical temperature profile of the liquid in the tank at the time of the measurement. Experimental measurements made in storage tanks show that a wide range of temperature profiles can exist (FIG. 1). Most ATGs use a calibration target located at a known distance from the transducer to estimate the propagation speed (i.e., sound speed) within a liquid medium; this is a widely accepted and widely published method, particularly in underwater sound measurements. The ATG makes an estimate of the average sound speed between the transducer and the surface from the time it takes for an acoustic pulse to travel round trip between the transducer and a solid reference target, such as fiducial 210, that is located a fixed and known distance above the transducer and below the liquid surface (FIG. 2); the time is then converted to distance. What is not included in this average sound speed is that portion of the liquid between the fiducial and the surface. Therefore, if the temperature in this layer differs from the average temperature in the layer between the transducer and the fiducial, errors in estimating the speed of sound through the liquid may occur, and this will affect the accuracy of the liquid-level estimate made by the system. If the acoustic transducer is located below the water/product interface, as shown in FIG. 3, the estimate of the sound speed obtained from the fiducial 210 will not reflect the propagation speed through product alone, but will include the effects of the water. (Accuracy also depends, of course, on the performance characteristics of the measurement system itself.)
An acoustic ATG is also used to measure the level of the water that accumulates at the bottom of a storage tank. It is generally understood that the maximum level of water that an ATG must be capable of measuring is 4 in. A higher water level is likely to interfere with or contaminate the liquid that is being dispensed from the tank. If water is immiscible with the liquid in the tank, which is the case with petroleum, the product most commonly stored in underground tanks, an acoustic ATG can be used to measure it. In principle, this measurement can be made in two ways. The first way is to position the transducer below the water/product interface and measure the round-trip travel time of the acoustic signal reflected from the interface (FIG. 4). The travel time is converted to distance by selecting an average value of sound speed through water from published tables. This method is more than sufficient for providing a level measurement accurate to within 1/8 in. For a number of reasons, this measurement is difficult to make if the water/product interface is too close to the transducer, and, as a consequence, this measurement approach has not been used commercially to measure the water level near the bottom of the tank. The second way is to position the transducer above the water/product interface and measure the difference in the travel time for an acoustic signal to propagate (1) from the transducer to the product surface to the water/product interface, then back to the surface and back again to the transducer (FIG. 5(a)); and (2) from the transducer to the product surface to the bottom of the tank, then back to the surface and back again to the transducer (FIG. 5(b)). The water level can be estimated from this first propagation path alone if the height of the transducer from the bottom of the tank is known and if the average sound speed over the entire propagation path can be estimated. Because temperature gradients are largest near the bottom and top of the liquid in the tank, errors in estimating sound speed occur in the liquid layer above the fiducial closest to the surface and the layer below the transducer. These errors can sometimes be large.
Leak detection is difficult because the thermal expansion and contraction of the volume of the product in the tank must be accurately estimated and removed from the volume changes derived from the level changes measured with an ATG. Accurate temperature compensation is difficult because the rate of change of temperature and the volume of the product in the tank, which is usually proportional to the circular cross-section of the tank, are not uniform with depth. Typically, when new product is delivered to an underground storage tank, there is a significant temperature difference between the new and extant products. This temperature difference arises because the product stored in an underground tank is likely to be in equilibrium (or nearly so) with the surrounding soil and backfill material, while the delivered product may have been transported in a tanker truck exposed to ambient air and sunlight conditions for a long time. Further, the tanker may have obtained the product from an above-ground tank, whose contents might have been much warmer (or colder) than the temperature of the ground where the receiving tank is located. When products of different temperatures are mixed, a thermal separation occurs with (usually, but not always) the warmer product rising to the top as the colder product settles to the bottom, with an infinite variety of different temperature profiles or "thermal gradients" between the top and bottom of the tank 10, such as illustrated in FIG. 1. Depending on the volume capacity of the storage tank, the thermal properties of the soil and backfill and the differences in temperature between the product in the tank and the soil and backfill, it can take many days for the combined products to reach near-equilibrium conditions again. In this attempt to reach a near-equilibrium condition, the rate of change of temperature may differ significantly over the depth of the tank. Because acoustic measurement systems are affected by the temperature over the entire propagation path between the transducer and the surface, they can be used to vertically integrate the changes in temperature over the depth of the product in the tank. When these measurements are made over a period of time, an acoustic system is particularly good for measuring the average change in temperature of the product within the tank.
An ATG can be used to conduct a volumetric leak detection test if both the average change in temperature of the product in the tank, which is weighted in the vertical by the volume of the product as a function of height above the bottom of the tank (i.e., by the cross-sectional area of the tank), and the change in level of the product can be measured over a period of time. The average thermally induced volume change is estimated by taking the mathematical product of the average volumetrically weighted temperature change, the coefficient of thermal expansion of the liquid, and the total volume of the liquid in the tank. When measurements are made in a partially filled tank, the average volume change is estimated from the average liquid-level change by means of a height-to-volume conversion factor determined from the tank geometry. (In an over-filled tank, volume change cannot be estimated from a height-to-volume conversion based on tank geometry, but must be measured experimentally.) The average temperature-compensated volume rate is calculated by subtracting the average thermally induced volume change from the average volume change. On the average, if the data are properly sampled to avoid aliasing the surface and internal waves that are frequently present in the tank, and if the liquid level changes due to the structural deformation of the tank and to the evaporation and condensation within the tank are also compensated for, this net volume change should, in a nonleaking tank, be equal to zero. The temperature-compensated volume rate is then compared to a predetermined threshold volume rate to determine whether the tank should be declared leaking. The performance of the leak detection system in terms of P.sub.D and P.sub.FA can be estimated if one generates a histogram of many individual leak detection tests on a nonleaking tank over a wide range of environmental conditions that affect the performance of the method, such as ground and product temperature conditions, and if one knows the relationship between the volume changes due to a leak and the volume changes due to any other physical mechanisms active in the tank environment. Acoustic ATG systems typically make an estimate of the average vertical temperature and average level change from a sound-speed estimate made with a fiducial located below the surface of the product. This fiducial is required so that one can compensate for the effects of temperature and level on the round-trip travel time of the acoustic pulse. When the average temperature change is estimated in this way, the temperature changes are not weighted by volume, and large temperature changes near the top and bottom of the tank, where the height of the product is great in comparison to its volume, can result in large differences between the depth-averaged temperature and the depth-averaged volumetrically weighted temperature. Therefore, the closer the fiducial is to the surface, the better the estimate of the average temperature and level changes will be. If the fiducial is located too far below the surface, it is likely that there will be large errors in the measurement of the liquid-level changes. Ideally, the fiducial should be collocated with the surface, but then it would be impossible to measure the acoustic returns from both the surface and the fiducial and to distinguish between them. Generally, a fiducial cannot be placed any closer to the surface than 1 to 2 in. This constraint is imposed by the width of the acoustic pulse, its reverberation, and the time required to process the data.
In U.S. Pat. Nos. 4,748,846 and 4,805,453, Haynes describes an ultrasonic ATG system and several methods for measuring the level of the product and water in a tank, a method for measuring the average sound speed through the product in a tank and the average temperature of this product, and a method for detecting theft or leaks in a tank. Haynes uses a number of fixed references or fiducials, rigidly and permanently attached to a staff that is inserted vertically into the tank, as shown in FIG. 2. The fiducials are separated by some predetermined distance, and preferably are equidistant from one another. The number of fiducials is not specified, nor is the spacing between them, but in a tank that is 8 ft in diameter there are typically 8 fiducials spaced approximately 12 in. apart. More fiducials can be used, but the spacing should not be so close that it becomes an intractable measurement problem to determine which acoustic return is associated with which fiducial or to determine which return is from the surface. Multiple returns from lower fiducials have round-trip travel times similar to the first return from higher fiducials, and the multiple returns from the fiducials and the surface have the same arrival times as the returns from the fiducials themselves. In addition, there is the possibility of missing a weak return from any one or more fiducials; this results in a counting (i.e., location) error. Furthermore, the minimum spacing, as determined from the duration of the acoustic pulse, reverberation, and processing time, is limited to 1 to 2 in. As a consequence, the fiducial closest to the product surface may be anywhere between 2 and 12 in. from it.
Haynes uses a single fiducial, the one that is closest to the surface, to measure product level and water level, to measure average temperature, and to make an estimate of the temperature-compensated volume for the purpose of leak detection. The transducer is located near the bottom of the tank, but above the maximum height of any water that might accumulate there. The system uses a threshold detection approach to measure the round-trip travel time of the acoustic signals reflected from all of the fiducials, the surface, the water/product interface, and the bottom of the tank. Haynes uses either of the configurations shown in FIG. 5 to estimate the water level; he states that when the water/product interface is close to the transducer, the configuration shown in FIG. 4 does not work.
There are a number of problems with the ATG system described by Haynes that affect the accuracy of the product-level and water-level measurements, as well as the leak detection test itself. All of the measurements require that both the surface and the fiducial immediately below the surface be identified. Multiple echoes can cause mistakes in finding the surface or the uppermost submerged fiducial. A secondary echo that has a round-trip travel time less than but very close to that of the first acoustic pulse may be detected instead of the first return of the pulse, resulting in an erroneous measurement of the distance between the transducer and the fiducial or surface. Since errors of 0.01 to 0.001 in. are significant, large errors can be made in estimating the sound speed, which is in turn used to make estimates of the product level, temperature, and water level. In general, a fiducial located within 12 in. of the surface will usually result in an estimate of the surface height accurate to within 1/8 in. or better. However, even a fiducial located an average of 6 in. away from the surface will produce unacceptably large systematic errors in the leak detection approach. Significant temperature gradients and, therefore, soundspeed gradients occur in the layer immediately below the surface, and thus placement of a fiducial more than 2 to 3 in. from the surface can result in large errors in the measurement of the level changes. In addition, an error in leak detection can occur because the temperature changes are not volumetrically weighted by the cross-sectional area of the tank. The approach to making water-level measurements that is shown in FIG. 5(a), where the pulse is reflected back to the transducer via the route surface-interface-surface, is susceptible to errors in that it is difficult to distinguish the interface return from the primary and secondary fiducial and surface returns. Errors in measuring the distance between the transducer and the interface are also likely to occur, because the speed of sound in the area between the transducer and the interface is not known, nor is the speed of sound between the uppermost submerged fiducial and the surface known.
In a number of other types of liquid-level measurement devices, floats have been used to track surface fluctuations. For example, U.S. Pat. No. 4,158,964, issued to McCrea et al., disclosed an ultrasonic level-measurement system that is very different from Haynes's acoustic system. In the McCrea invention, an ultrasonic pulse is transmitted vertically through a waveguide made of a homogeneous aluminum alloy having a low thermoelastic coefficient. Permanent magnets are located at the top and bottom of the waveguide and on a donut-shaped float that is concentrically positioned about the waveguide and that is free to move up and down with liquid-level changes. These magnets produce a low-level, bipolar voltage pulse that is generated across the waveguide as the acoustic pulse transmitted along the waveguide passes the magnet. The permanent magnets at the top and bottom of the tube are used as a calibration reference to interpret the speed of the pulse in the waveguide. The magnets in the float are used to determine the position of the float along the waveguide.